A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a pattern provided by a patterning device (e.g., a mask pattern or a reticle pattern) may be used to generate a circuit pattern to be formed on an individual layer of the IC. The circuit pattern can be transferred onto a target portion (e.g. comprising part of one or more dies) on a substrate (e.g. a silicon wafer). Transfer of the circuit pattern is typically via imaging of the pattern onto a layer of radiation-sensitive material (resist) provided on the substrate, using a projection system. A beam of radiation is patterned by having that beam traverse the patterning device, and is projected by the projection system onto a target portion on a substrate (silicon wafer) that has been coated with a layer of photo-activated resist (i.e., photoresist) material, such as to image the desired pattern in the resist. A lithographic printing process further includes a development of the resist layer after exposure such as to generate printed features, which may be features of resist material, or spaces in resist material. The resist material may serve as etch mask for an underlying layer to be patterned by etching.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed to patterned radiation. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate substantially parallel or anti-parallel to this direction.
In the semiconductor industry, the continual demand for smaller semiconductor devices, having smaller patterns and features on the substrate, is pushing a requirement for improved control of a critical dimension (CD) of a printed feature.
Generally, a critical dimension of a feature of a pattern printed on the substrate depends on attributes of the projection system, the (patterned) beam of radiation, the pattern and the resist processing (including for example a pre- and post-exposure bake process, and a resist development). An attribute of a line feature of the pattern is for example the direction of the line. For example, the pattern may include horizontal and vertical line features, i.e., features having a line-shaped portion including a line segment respectively substantially perpendicular to the scanning direction and substantially parallel to the scanning direction of a scanner (or, when a stepper is used, respectively substantially perpendicular and substantially parallel to a side of a die as printed on the substrate). A desired line width of the line segment of both horizontal and vertical lines may be the same, however, the respective printed line width of the horizontal and vertical lines may be different, the difference generally referred to as a H-V difference.